I have 2 red eared sliders in a 20 gallon (76 liters) tank and I got them both 7 months ago. They were both the size of a 50 cent coin (about 3 cm) and the one hasn't grown past that size and only eats a couple bites of food. On the other hand, my second turtle is now 3 times the size and eats a lot. I just did research - I may be overfeeding him, but my little guy hasn't grown at all.
I am not sure what to do. Please let me know what advice you have.